1. Field of the Invention
The present invention relates to a rare earth bonded magnet which is produced such that a rare earth magnetic powder as a principal component is combined with a binding resin (bond resin), and particularly to a rare earth bonded magnet which is formed by compression molding, incorporated in a rotary device, such as a motor, and which is required to be heat resistant, durable and weather resistant in a hot environment.
2. Description of the Related Art
A rare earth permanent magnet has excellent magnetic properties and therefore is extensively used, typically in rotary devices or elements, and also in general home electric appliances, audio equipment, medical equipment, general industrial instruments, and the like. Especially, a rare earth bonded magnet, which is formed of a rare earth magnetic powder combined with a binding resin, is highly flexible in formation and so helps reducing the size and enhancing the performance in the usage application described above. The molding methods for a rare earth bonded magnet include compression molding, injection molding, extrusion molding, and the like, wherein the kind of resin used varies according to the molding method employed. Generally, the resin to be used is selected according to the application of a permanent magnet, specifically such that a thermosetting resin is used for compression molding while a thermoplastic resin is used for injection molding and extrusion molding. A rare earth bonded magnet, which is made by compressing molding using a thermosetting resin, can be configured to contain an increased amount of magnetic powder inside a resultant permanent magnet thus realizing a permanent magnet provided with enhanced magnetic properties.
A rare earth permanent magnet may further be used in vehicles, more typically in automobiles, in addition to the aforementioned application areas. Conventionally, a ferrite permanent magnet that is excellent in heat resistance, durability and weather resistance has been used in the automotive application. In the meantime, since output increase and size reduction have been increasingly called for, a permanent magnet with a high surface magnetic flux is becoming necessary and therefore a magnet material with excellent magnetic properties is required. Thus, a rare earth permanent magnet is more and more often used.
In the automotive application, the usage environmental conditions are severe compared to in the other applications described above. Specifically, the permanent magnet is used, for example, at a temperature below the freezing point and also in a high heat environment in the vicinity of an engine room. Thus, the usage temperature is assumed to range very widely. Also, because of usage in all kinds of weather, fine and rainy days, the automotive application must work in a wide range of humidity. For this reason, it is required that a permanent magnet material intended for the automotive application enables the resulting permanent magnet to maintain adequate magnetic properties over a long period of time in a wide range of usage temperature and humidity. That is to say, the permanent magnet material for the automotive application must be so constituted that the resulting permanent magnet undergoes only limited demagnetization under environmental changes and is heat resistant, durable and weather resistant.
The ferrite permanent magnet used conventionally for the automotive application is an oxide, therefore is chemically stable and not demagnetized in a high temperature state. At a temperature below ordinary temperature, however, the ferrite permanent magnet undergoes a phenomenon called “low-temperature demagnetization”, and it is difficult to achieve desired motor characteristics (for example, rotary torque) when used in low temperature environment. On the other hand, the rare earth permanent magnet, though free from demagnetization at low temperatures, undergoes a considerable variation in magnetization and coercive force depending on temperature and experiences a decrease in its magnetization with increasing temperature. Further, the rare earth permanent magnet, when exposed to a high temperature for a long time, exhibits a time-dependent variation in magnetization, thus causing so-called “thermal demagnetization”.
Also, a rare earth magnet material is an alloy (so-called “metal alloy) and therefore easily becomes oxidized if oxygen comes into contact with the surface. The thermal demagnetization of a permanent magnet falls into two types: one is permanent demagnetization caused by organizational change such as oxidation in the magnet material itself; and the other is irreversible demagnetization unrelated to organizational change. For example, in a rare earth permanent magnet composed principally of neodymium (Nd), iron (Fe), boron (B) and the like, Nd2Fe14B as a main phase as well as grain boundary phases (Nd-rich phase and B-rich phase) present around main phase crystal grain are characteristically susceptible to oxidation. If the phases undergo organizational change due to oxidation and the like, the magnetic properties such as magnetization, coercive force (HcJ) and demagnetization curve squareness are deteriorated. The magnetic properties once deteriorated cannot be recovered by re-magnetization, which significantly influences and deteriorates the performance characteristics (for example, rotary torque) of a rotary device such as a motor. Accordingly, the technique of preventing the oxidation of the rare earth permanent magnet helps reducing the permanent demagnetization and constituting an important factor to determine the properties of a magnet and also the characteristics of a rotary device such as a motor.
Coating the surface of a magnet is a typical technique for preventing the oxidation degradation of the rare earth permanent magnet. The rare earth bonded magnet is coated with resin by an electro-deposition painting method, a spray painting method, or the like. Oxygen and moisture contained in the open air are prevented from contacting the surface of the magnet or invading in the magnet, if the surface of the magnet is coated with resin.
However, it is difficult to completely cover the entire surface of the magnet by such resin coating methods, so that it happens that the resin coating layer includes unpainted areas at contact marks with a coating tool, so-called pin holes, and like defects. Air and moisture easily pass through the pin holes, which triggers the oxidation degradation of the magnetic material. As a result, a sufficient durability cannot be effectively achieved. Also, voids may be present inside the magnet, and air existing in the voids and containing oxygen may possibly get into touch with magnetic powder. Especially, the rare earth bonded magnet, which is made by compression molding, often includes, other than magnetic powder and binding resin, 10% or more voids, which gives good chances of oxygen getting into touch with magnetic powder.
For this reason, in order to efficiently prevent the magnetic powder from getting into touch with oxygen and moisture, unconventional measures must be taken, specifically such that individual magnetic powders are coated with resin or the like, or a surface treatment is applied to individual magnetic powders. Methods of coating individual magnetic powders or applying a surface treatment thereto as described above are disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-244106, Japanese Patent Application Laid-Open No. H6-349617, Japanese Patent No. 3139826, Japanese Patent Application Laid-Open No. 2003-86411, and Japanese Patent No. 3882545.
Japanese Patent Application Laid-Open No. 2001-244106 relates to a rare earth magnetic powder to which a surface treatment is applied, and also relates to a method of applying such a surface treatment, wherein it is proposed that the surface of a rare earth magnetic powder is treated with phosphonate salt so as to form an antioxidant coating on the surface to thereby prevent rusting and oxidation, and that a bonded magnet is produced in such a way that the magnetic powder subjected to such a surface treatment is mixed with resin and molded using an injection molding machine. Japanese Patent Application Laid-Open No. H6-349617 discloses a bonded magnet which is produced such that binding resin is selectively mixed with an organic phosphorous compound thereby enabling suppression of oxidation degradation in the binding resin, and thus which is excellent in corrosion resistance and mechanical strength and therefore is adapted to maintain a high reliability over a long period of time.
Japanese Patent No. 3139826 discloses a bonded magnet which includes a rare earth-iron-nitride-based material, is excellent in magnetic properties and oxidation resistance and which is produced such that a rare earth-iron-nitride-based magnetic powder, anti-oxidizing agent and thermosetting resin are mixed together, wherein the anti-oxidizing agent includes an organic phosphorus compound, whereby oxidation resistance is achieved even in high temperature environment.
Japanese Patent Application Laid-Open No. 2003-86411 discloses a bonded magnet which is produced such that a curing reactive silicone rubber is used as binding resin, and that magnetic powder is coated with an inorganic phosphorus compound and a coupling agent, whereby the anti-rusting properties are enhanced. And, Japanese Patent No, 3882545 discloses a bonded magnet which is produced such that a uniform phosphate coating is formed on the surface of an iron-based magnetic powder containing a rare earth element, wherein the function and the configuration of the phosphate coating are optimized, whereby the bonded magnet is made excellent in weather resistance.
However, the above conventional arts have the following problems in terms of heat resistance, durability and weather resistance. Japanese Patent Application Laid-Open No. 2001-244106 teaches that phosphonate salt is used as surface treating agent, wherein the phosphonate salt is caused to exude due to moisture in the air thereby protecting the magnetic powder. The phosphonate salt functions as a chelating agent and accelerates the passivation tendency of a metal surface. The above function is effective in forming an anti-oxidant coating for a bonded magnet used by itself alone, but when the magnet is used in combination as a constituent component in a rotary device, it is very probable that the phosphonate salt exudes out of the magnet due to moisture in the air, in which case if the phosphonate salt performs chelating function on other constituent components, especially metallic components, then an influence on the components and external apparatuses is inevitable.
Japanese Patent Application Laid-Open No. H6-349617, while describing that binding resin is selectively mixed with an organic phosphorous compound thereby producing a bonded magnet excellent in corrosion resistance and mechanical strength, does not discuss thermal demagnetization of a permanent magnet used in high temperature environment. Also, though it is described therein that corrosion is caused by chlorine which is produced such that halide ion, especially chloride ion contained in epoxy resin reacts with moisture in the air, no solution is given to thermal demagnetization of a permanent magnet used in high temperature environment.
Japanese Patent No. 3139826 states that a bonded magnet which exhibits oxidation resistance even in high temperature environment is realized in such a manner that earth-iron-nitride-based magnetic powder, anti-oxidizing agent and thermosetting resin are mixed together, wherein the anti-oxidizing agent includes an organic phosphorous compound. The description therein, however, is made specifically on a magnet material formed of a rare earth-iron-nitride-based magnetic material and fails to discuss the heat resistant effect of a rare earth magnet material composed principally of an arbitrary substance, among others, neodymium (Nd), iron (Fe) or boron (B). Also, Japanese Patent No. 3139826, while showing a solution to the oxidation degradation of a magnetic powder during processing, does not teach a solution to the thermal demagnetization of a permanent magnet used in high temperature environment and fails to provide an effect of reducing the deterioration of magnetic properties, such as magnetization, coercive force (HcJ) and demagnetization curve squareness, wherein the deterioration is caused due to usage in high temperature environment.
Japanese Patent Application Laid-Open No. 2003-86411 characteristically states that a magnetic powder is coated with an inorganic phosphorus compound and a coupling agent, a curing reactive silicone rubber is used as binding resin, whereby the anti-rusting properties are enhanced. However, the additive amount of the silicone rubber-based binder is 10 to 20 weight portion against 100 weight portion of the magnetic powder and therefore it is difficult to obtain a high density compact. As a result, it is difficult to achieve desired magnetic properties.
Japanese Patent No. 3882545 characteristically teaches a bonded magnet which is produced such that a uniform phosphate coating is formed on the surface of an iron-based magnetic powder containing a rare earth element, wherein the function and the configuration of the phosphate coating are optimized, whereby the bonded magnet becomes excellent in weather resistance. However, the binder discussed therein is a thermoplastic resin, and it is not demonstrated if a comparable effect is achieved when any alternative binder, for example a thermosetting resin, is used. Further, most of thermoplastic resins, such as polyamide, can be used continuously at an ambient temperature of not more than about 100 degrees C., and therefore it is difficult for a bonded magnet including such a resin to be used continuously in a higher temperature environment.
Moreover, all of the conventional arts described above fail to expressly discuss the deterioration of the squareness ratio (Hk/HcJ). When a rare earth magnet is oxidized and degraded, the coercive force is lowered and at the same time the squareness is deteriorated. If the magnet is used by itself alone, it is good enough to cope solely with the lowering of the coercive force. But, when the magnet is used in combination as a constituent component of a magnetic circuit, for example, in a motor, if the squareness ratio (Hk/HcJ) decreases considerably, then a magnetic flux generated from the magnet may possibly be decreased. So, it is required to minimize the deterioration of the squareness.